This invention relates to an improved focus-mask-type CRT (cathode-ray tube) and to a method for operating this improved CRT.
A commercial shadow-mask-type color television picture tube, which is a type of CRT, comprises generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in cyclic order, means for producing three convergent electron beams directed towards the target, and a color-selection structure including an apertured masking plate between the target and the beam-producing means. The masking plate shadows the target and, therefore, is also called a shadow mask. The differences in convergence angles permit the transmitted portions of each beam, or beamlets, to select and excite phosphor elements of the desired emission color. At about the center of the color-selection structure, the masking plate of this commercial CRT intercepts all but about 18% of the beam currents; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the mask. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures with respect to the area of the plate, without substantially increasing the excited portions of the target area. In one approach, the apertures are arranged in columns opposite substantially parallel phosphor stripes in the target. Each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor. The two beamlets passing through adjacent windows are deflected towards one another, and both beamlets fall on substantially the same area of the target. In this approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One family of CRTs employing such a combined deflection-and-focus color-selection means includes, as normally viewed, a target comprised of a mosaic of vertical phosphor stripes of three different emission colors arranged cyclically in triads (groups of three different stripes), means for producing three convergent horizontally in-line electron beams directed towards the target, and a color-selection structure located adjacent and closely spaced from the target. The color-selection structure comprises a metal-masking plate having therein an array of substantially rectangular apertures arranged in vertical columns and a single array of narrow vertical conductors in the form of wires insulatingly spaced and supported from one major surface of the masking plate, with each wire conductor substantially centered over the apertures of one of the columns of apertures. Each wire conductor is unsupported and uninsulated over each aperture. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows.
When operating this latter device, the narrow vertical conductors are electrically biased with respect to the masking plate, so that the beamlets passing through each of the windows of the same aperture are deflected horizontally toward the positively-biased side of the window. Simultaneously, because of quadrupole-like focusing fields established in the windows, the beamlets are focused (compressed) in one direction of the phosphor stripes and defocused (stretched) in the other direction of the phosphor stripes. The spacings and voltages are so chosen to form an array of electrostatic lenses that also deflects adjacent pairs of beamlets to fall on the same phosphor stripe of the target. The convergence angle of the beam that produces the beamlet determines which stripe of the triad is selected.
This color-selection structure requires electrical insulation between the masking plate and the wire conductors that comprise the color-selection structure. In such structures that have been made up to the present, some insulation is left, after all fabricating processes have been completed, in positions where it is exposed to electron bombardment. This bombardment electrostatically charges surfaces of the insulator with a resultant severe distortion of the final beam spot. Although heroic measures, such as sandblasting and spot-knocking, achieve some success in removing exposed insulation, these are not practical, large-scale remedies for mass producing this structure.